The term "flowing substance" occurring hereinafter is intended to comprise various liquids, suspensions etc containing constituents which are desirable to filter away. Even if this invention is in no way restricted thereto, a preferred use of the invention is in connection with dewatering of fibre suspensions. Cellulose fibres are primarily intended as fibres although also other applications are possible. In case of fibre suspensions, the invention may for instance be used for filtration of back water in paper machines.
Although the invention with preference is useful in so called disc filters and hereinafter will be more specifically described in connection with such a filter, it is pointed out that the invention can also be used with other types of filters of a rotating nature.
A relatively cloudy filtrate, pre-filtrate, is obtained in the first filtration region. The filter element arrives in this first filtration region liberated from material layers previously deposited thereon. The openings of the filter element are, thus, open in the beginning of the first filtration region, which initially means a substantial passage of fibres so that the pre-filtrate will be "cloudy", i.e. contain relatively high contents of fibre. A fibre layer is successively built up in the first filtration region, a fact that will successively improve the filtering ability, i.e. reduce the fibre contents in the filtrate.
Rotating filters of the kind intended here occur in two versions. In a first of these, a substantially atmospheric pressure is used in the first filtration region, i.e. vacuum is not used to promote pre-filtrate passage through the filter element. Thus, it is only gravity that influences the pre-filtrate to pass through the filter element and the fibre layer deposited thereon. This means that the filtering capacity expressed as volume per time unit becomes comparatively low. In addition, it is pointed out that the fibre layer deposited on the filter element will be rather porous. When the fibre layer during rotation of the rotor arrives in the second filtration region, the clear filtrate region, a vacuum is applied which influences the filtrate through the fibre layer and the filter element and this vacuum will tend to cause a chock which gives rise to a troublesome passage of fibres which will join the clear filtrate.
In the second of the filter versions, a vacuum is used not only in the second filtering region but also in the first to increase the filtering efficiency expressed as the volume of substance per time unit as compared to the case where an atmospheric dewatering pressure is used. This increased efficiency is, however, achieved in connection with an inconveniently high initial passage of fibres, which gives rise to a high amount of fibre in the pre-filtrate. The fibre contents will be inconveniently large also in the clear filtrate as compared to the case where atmospheric dewatering pressure is used in the first filtration region, which is probably due to the fact that application of a vacuum in the first filtration region does not allow build up of a fibre mat on the filter element, said fibre mat operating efficiently thereafter from a filtration point of view.
Even if, as pointed out hereinabove, the clear filtrate has turned out to have, when vacuum is used in the first filtration region, a higher proportion of fibres than is the case when atmospheric pressure is used in the first filtration region, it can be established that also when atmospheric pressure is used, the clear filtrate has an inconveniently high proportion of fibres. The consequence thereof has been that one often has had to couple two filters in series to achieve an acceptably low proportion of fibres in the clear filtrate. In this connection it is pointed out that the pre-filtrate at least in some applications is recirculated to the filter inlet. This means that it is of course also important to reduce the fibre contents in the pre-filtrate.
The object of the present invention is to devise ways to develop prior rotating filters for the purpose of reducing the disadvantages discussed hereinabove. In a corresponding manner, the invention aims at improving the filtration method such that an improved filtration result may be achieved.
By designing the first filtration region such that at least two filtration zones having different pressure relations are established in said first filtration region, a substantially improved filtration result is achieved in the sense that the contents of the material, in particular fibres, which is to be filtered off is reduced at least in the clear filtrate. Besides, the filtering capacity expressed as volume substance to be filtered per time unit is also improved.
The explanation of the improved filtering function is that an atmospheric pressure or possibly a relatively weak vacuum is used in the first filtration zone in the first filtration region so that formation of a comparatively porous material layer on the filter element is allowed. Not until in a subsequent second filtration zone contained in the first filtration region vacuum is applied. Since there is already in this second filtration zone a filtering material layer on the filter element, this material layer will function filtrating comparatively efficiently on application of a vacuum. The increased value of the filtrate passage as a consequence of the vacuum applied causes, accordingly, a considerable capacity increase in the first filtration region compared to the case where vacuum is not applied in this first filtration region. When the vacuum is applied, a compressing will occur of the material layer deposited on the filter element so that the filtration ability is then considerably improved as far as separation is concerned compared to the two different prior filter versions discussed hereinabove.
An important feature of the present invention is, accordingly, that the first pressure change occurring on filtration between a relatively high pressure, for instance atmospheric pressure or a relatively weak vacuum, and a relatively low pressure, namely a vacuum, occurs within the first filtration region, i.e. within the region where the pre-filtrate is separated, such that the increased amount of material to be filtered off and which will be discharged in the filtrate as a consequence of the pressure change will end up in the pre-filtrate and not in the clear filtrate. It is in this connection emphasised that what is here intended is the "first" pressure change occurring within the first filtration region, a fact which does not at all preclude that after this first pressure change one or more further pressure changes are conducted. Thus, a pressure change in this regard is a change of the pressure drop over the filter element in the filtrate passage direction in an increasing sense by decreasing the pressure on the downstream side of the filtering surface.